This invention relates to a process for the production of amorphous and/or crystalline oxides of metals of the third to fifth main group or the secondary groups which have mean particle diameters in the nanometer range. In the context of the invention, this is the range from about 1 to about 500 nanometers. More particularly, the amorphous or crystalline metal oxides obtainable by the process according to the invention have particle diameters in the range from about 5 to about 100 nanometers. Metal oxides such as these may be used for various industrial applications: as dielectrics for miniaturized multilayer capacitors, as catalysts, as additives in paints and cosmetics, as additives in plastics to stabilize them against thermal or photochemical decomposition and/or to modify their dielectric and/or magnetic properties and as polishes.
Metal oxides with particle diameters in the nanometer range may be obtained, for example, by dissolving alkoxides of the metals in a water-immiscible solvent, preparing an emulsion of the resulting solution in water using suitable surfactants, the emulsified droplets of the solvent having diameters in the nanometer range, and hydrolyzing the metal alkoxides to the oxides. The disadvantages of this process lie in particular in the fact that the metal alkoxides are expensive starting materials, in the fact that emulsifiers also have to be used and in the fact that the preparation of the emulsion with droplet sizes in the nanometer range is a complicated process step.
It is also known that metal particles (not metal oxide particles!) with a particle size below 30 nm can be produced by cathodically reducing suitable metal salts in organic solvents or mixtures thereof with water in the presence of a stabilizer and optionally in the presence of a supporting electrolyte. Instead of dissolving metal salts in the electrolyte, the metal ions to be cathodically reduced can also be dissolved by using anodes of the corresponding metals which dissolve during the electrolysis. One such process is described in DE-A-44 43 392.
In addition, DE-A44 08 512 describes a process for the electrolytic production of metal colloids in which one or more metals belonging to groups IV, VII, VII and I.b of the periodic system are anodically dissolved in aprotic organic solvents in the presence of a supporting electrolyte and cathodically reduced in the presence of stabilizers to colloidal metal solutions or redispersible metal colloid powders with a particle size below 30 nm. The supporting electrolyte and the stabilizer may be identical. If the cathodic reduction is carried out in the presence of suitable supports, the metal colloids are precipitated onto those supports.
In addition, according to Chemical Abstracts Report 110:65662, fine-particle zirconium oxide powder can be obtained by electrochemically producing a base in a solution of zirconyl nitrate, the zirconyl nitrate being hydrolyzed by the base with precipitation of hydrated zirconium oxide. Crystalline zirconium oxide can be obtained from the hydrated zirconium oxide by calcination. According to Chemical Abstracts Report 20 114:31881, mixed oxides of iron, nickel and zinc can be produced by electrochemically precipitating a hydroxide mixture of those metals from metal salt solutions and calcining the isolated hydroxides to the mixed oxides.
The problem addressed by the present invention was to provide a new process for the production of amorphous and/or crystalline oxides of metals or mixed oxides of several metals which have mean particle diameters of about 1 to about 500 nm.
Accordingly, the present invention relates to a process for the production of amorphous and/or crystalline oxides of metals of the third to fifth main group or the secondary groups of the periodic system which have mean particle diameters in the range from 1 to 500 nm, characterized in that, using a cathode and an anode, ions of those metals dissolved in an organic electrolyte are electrochemically reduced at the cathode in the presence of an oxidizing agent.
FIGS. 1 to 3 are X-ray diffractograms of certain metal oxide samples produced in the working examples of the present invention.
In this process, the mean particle diameter can be adjusted by varying the temperature of the electrolyte or the electrical voltage or current intensity or through the nature of the supporting electrolyte optionally used. The process is preferably carried out in such a way that the metal oxides obtained have mean particle diameters in the range from 5 to about 100 nm.
Using this process, it is only possible to produce metal oxides which do not react with moisture to form hydroxides at a temperature below about 100xc2x0 C. Accordingly, the process is not suitable for the production of oxides of alkali or alkaline earth metals. It is particularly suitable for the production of oxides of metals which are oxidized by atmospheric oxygen at temperatures below about 100xc2x0 C. Where metals such as these are used, the process according to the invention may be carried out at temperatures below 100xc2x0 C. using air as the oxidizing agent. This enables the process to be carried out in an uncomplicated manner. The process is particularly suitable for the production of amorphous and/or crystalline oxides of Ti, Zr, Cr, Mo, Fe, Co, Ni and Al.
The organic electrolyte used is preferably a substance which is liquid at temperatures in the range from about xe2x88x9278xc2x0 C. to about +120xc2x0 C. at normal pressure. In one particularly preferred embodiment, a substance which is liquid at temperatures in the range from about 0 to about 60xc2x0 C. at normal pressure is used. The organic electrolyte is preferably selected from alcohols, ketones, ethers, nitrites and aromatic compounds, those which are liquid at temperatures in the ranges mentioned being preferred. Particularly suitable electrolytes are tetrahydrofuran, acetone, acetonitrile, toluene and mixtures thereof with alcohols.
Depending on the metal oxide to be produced, it can be favorable if the electrolyte contains small quantities of water. For example, the water content of the organic electrolyte may be in the range from about 0.01 to about 2% by weight and, more particularly, is in the range from about 0.05 to about 1% by weight, the percentages by weight being based on the total quantity of organic electrolyte and water.
Should the electrolyte not of itself have an adequate electrical conductivity or acquire an adequate electrical conductivity by dissolution in salts of the metals whose oxides are to be produced, it is advisable to dissolve a supporting electrolyte in the electrolyte. The usual supporting electrolytes which are normally used to give the electrolytes mentioned an electrical conductivity sufficient for electrochemical processes may be employed. Suitable supporting electrolytes are, for example, electrolyte-soluble hexafluorophosphates, sulfonates, acetyl acetonates, carboxylates and in particular quaternary phosphonium and/or ammonium salts with organic groups at the phosphorus or at the nitrogen. Preferred supporting electrolytes are quaternary ammonium compounds which bear aryl and/or alkyl groups at the nitrogen and which are preferably present as halides. A particularly suitable example is tetrabutyl ammonium bromide.
The process according to the invention is preferably carried out in a temperature range in which the supporting electrolyte is sufficiently soluble in the organic electrolyte. The process is preferably carried out in such a way that the organic electrolyte has a temperature in the range from about 30 to about 50xc2x0 C. If tetrahydrofuran is used as the electrolyte and tetrabutyl ammonium bromide as the supporting electrolyte, the process is preferably carried out at temperatures above 35xc2x0 C. for example in the range from 35xc2x0 C. to 40xc2x0 C.
The supporting electrolytes have the additional effect that they protect the oxide particles formed against agglomeration. A very narrow particle size distribution can be obtained in this way. As described in a following Example, zirconium dioxide with a volume-averaged crystallite size of 8 nm, for example, can be produced in this way, the particle size distributionxe2x80x94expressed as the logarithmic normal distributionxe2x80x94having a sigma value of 1.29. If no importance is attached to a narrow particle size distribution, there is no need to add the supporting electrolytes providing the electrolyte has an adequate electrical conductivity from the dissolved salts of the metal to be precipitated as oxide.
According to the invention, the metal oxides are formed by electrochemical reduction of the ions of the metals at a cathode in the presence of an oxidizing agent. The easiest oxidizing agent to use is oxygen or air. Accordingly, oxygen or air is preferably used. In a preferred embodiment, therefore, the process is carried out by introducing air into the electrolyte during the electrochemical reduction of the metal ions. If desired, oxygen-enriched air or substantially pure oxygen may also be introduced into the electrolyte. Other suitable but less preferred oxidizing agents are hydrogen peroxide, organic or inorganic peroxo compounds or oxo anions of the halogens chlorine, bromine or iodine where the halogen has an oxidation number of +1 to +5. However, if stronger oxidizing agents than atmospheric oxygen are used, it is important to ensure that there is no peroxide formation with the electrolyte.
The electrical d.c. voltage between cathode and anode is preferably adjusted so that, for an anode area and a cathode area of 800 mm2, a current flow of the order of about 5 to about 100 mA and more particularly in the range from about 10 to about 50 mA occurs. Given a sufficiently conductive electrolyte, this can be achieved by applying a d.c. voltage of about 1 to about 100 volts between the cathode and anode.
In a preferred embodiment, the electrolyte is vigorously agitated throughout the process. Thus can be done, for example, by stirring the electrolyte. In addition or alternatively, the electrolyte may be ultrasonicated for this purpose. The advantage of electrolyte agitation and/or ultrasonication is that the metal oxides formed do not adhere to the cathode and cover it with an insulating layer.
The ions of the metal or metals whose oxides or mixed oxides are to be produced can enter the electrolyte in various ways. For example, it is possible to use an anode which contains the metal whose oxide is to be produced and which dissolves anodically during the production of the oxides. Accordingly, the anode may consist, for example, of the metal whose oxide is to be produced. Alternatively, an anode of an inert material coated with the metal whose oxide is to be produced may be used. In the latter case, the corresponding metal separates anodically from the anode during the electrochemical production of the metal oxide.
In cases where it is desired to produce an oxide of only one metal, an electrode consisting solely of, or coated with, that metal is used. However, mixed oxides of various metals can also be produced by the process according to the invention. In this case, either an anode consisting of, or coated with, these various metals may be used or, alternatively, several anodes each consisting of or coated with a different metal may be used. The alternative procedure has the advantage that different voltages may be applied between the cathode and the various anodes in order to take account of the different solution potentials of the various metals.
However, the process according to the invention may also be carried out by using an inert anode and dissolving in the electrolyte a salt of the metal or salts of the metals of which the oxide or mixed oxide is to be produced. In this case, the salts selected must of course be sufficiently soluble in the electrolyte used. Where tetrahydrofuran is used as the electrolyte, chlorides or nitrates of the particular metals, for example, are generally suitable.
A material which is inert under the electrolysis conditions selected is preferably used as the cathode material and optionally the anode material. Suitable electrode materials are, for example, electrodes of platinum or other platinum metals, gold, stainless steel, titanium or glassy carbon.
The oxides or the mixed oxides are obtained in X-ray amorphous or crystalline form, depending on the metal and the electrolysis conditions. Accordingly, they show either an X-ray diffractogram which resembles that of a liquid and has only a few broad maxima (X-ray amorphous, FIG. 1) or which consists of individual clearly contrasting X-ray reflexes (X-ray crystalline, FIG. 2, FIG. 3). The X-ray amorphous or X-ray crystalline metal oxides obtained are separated from the electrolyte either continuously or in batches, for example by continuous or discontinuous filtration or centrifugation. If necessary, the metal oxides separated from the electrolyte are washed, preferably with the organic solvent used as electrolyte, in order to remove any salt residues present. The metal oxides are then dried, for example at a temperature of 100xc2x0 C.
If it is intended to produce crystalline metal oxides or mixed oxides and if they do not. accumulate in the. desired form during the electrolysis process, the metal oxides separated from the electrolyte may be thermally aftertreated. For example, they may be converted into an X-ray crystalline form by calcination at a temperature in the range from about 300 to about 1200xc2x0 C. and more particularly at a temperature in the range from about 400 to about 1,000xc2x0 C. The calcination time will depend on the rate at which the amorphous oxides are converted into the crystalline oxides and may be, for example,.between about 5 minutes and about 4 hours. Depending on the metal oxide selected, the size of the crystallites may increase with increasing calcination time.